Microtubules (MTs) are composed of alpha and beta tubulin subunits in a 1:1 ratio (1,2). The alpha and beta subunits are similar in size (55,000 MW) and amino acid composition. The morphological 4 x 5 nm subunit seen in electron micrographs of the microtubule wall corresponds in size to a single 55,000 MW subunit, so an alternating arrangement of alpha and beta subunits has been proposed. In many cases there is no obvious structural difference in alternate subunits which might be expected since the subunits are so similar. In other cases a weak 8nm layer line in x-ray diffraction and in optical diffraction of electron micrographs has indicated a structural difference in alternate subunits. It should be noted that, although it seems reasonable to attribute the 8nm periodicity to structural difference of the alternate alpha and beta tubulin the subunits have not really been identified as such. It is possible that the structural differences are due to the presence of non-tubulin material, such as the Microtuble associated proteins (MAPs) that are present in most of these specimens. What we need is a positive and direct identification of the two types of subunits.